Coating



Patented July 10, 1934 UNITED STATES PATENT OFFICE coa'rmo No. Drawing.

6 Claims.

This invention relates to the coating of aluminum and aluminum base alloys, herein and in the appended claims collectively defined as aluminum, with an oxide coating which, in the parll lance of the art, is a coating artificially produced on aluminum and consisting in substantial part of aluminum oxide. The invention particularly relates to the formation of such coatings on aluminum by the use of solutions of oxalic acid of certain concentration as electrolytes in electrolytic cells in which the aluminum to be coated is made the anode.

Solutions of oxalic acid have been proposed in the art as electrolytes in the anodic processes of providing aluminum with an oxide coating, but

their use has not been extensive because of cer-' tain difllculties, both chemical and economic, inherent in the solutions heretofore proposed. Such proposed solutions contain 1.0 to 3.0 per cent by weight of oxalic acid.

When the oxalic acid concentration is within the range of 1.0 to 3.0 per cent by weight, the anodic coating of aluminum in such solutions proceeds with difliculty. The metal is generally pitted during coating and this pitting cannot be overcome in any substantial degree without the use of alternating current alone or superposed on direct current. This has several disadvantages in commercial work. The electrical equipment is relatively complicated and expensive, and alternating current does not form the coating as quickly as direct current, thus reducing current erliciency. Likewise successful operation of the anodic process in solutions containing 1.0 to 3.0 per cent of oxalic acid, when direct current is employed, requires an operating voltage of about 60 to 100 volts with the expense attendant on the use of such high voltages.

I have discovered that these diflicultles, i. e., pitting of the metal, use of alternating current, or the operation at high voltages with direct current, can be overcome by the use of oxalic acid solutions containing 5.0 to 9.0 per cent by weight of acid. When solutions of this concentration are used as the electrolyte in an electrolytic cell in which aluminum is the anode, I have found that a satisfactory oxide coating is formed on the aluminum by the use of direct current at operating voltages below 50 volts, preferably about 25 to 45 volts, and this without any material pitting of the aluminum.

In addition, I have found, in the use of solutions of 5.0 to 9.0 per cent of oxalic acid for the purpose described, another entirely unexpected and 55 material advantage. In most anodic processes Application October 5, 1932, Serial No. 636,363

of forming oxide coatings on aluminum, it is preferable, if not necessary, to the formation of a good coating to maintain the electrolyte at a relatively low temperature, usually around 30 centigrade. This is true where oxalic acid solu- 00 tions are used as electrolyte. It is apparent that at certain times, especially in the warmer months of the year, the room temperature may equal or exceed this value. As a consequence, external cooling of the bath must be provided. In com- 05 mercial operation expensive mechanical refri eration is used. I have discovered that the use of solutions containing 5.0 to 9.0 per cent by weight of oxalic acid reduces the amount of refrigeration necessary and materially reduces, therefore, the cost of operation. Exemplary of this fact is the following: Two cells, one contain ing a 3 per cent solution of oxalic acid, the other containing a 9 per cent solution of oxalic acid, were connected in series so that the identical current passed through each cell. Aluminum was made the anode of each cell. The cells were not refrigerated but were placed in a water bath. Current was passed for one hour. During this period the temperature in the cell containing the 3 per cent oxalic acid electrolyte rose from 71 Fahrenheit to 106 Fahrenheit. In the same period the temperature in the cell containing the 9 per cent oxalic acid solution rose from 71 Fahrenheit to only 96 Fahrenheit. This indi- B5 cates definitely the saving in refrigeration costs which can be effected by the use of electrolyte containing 5.0 to 9.0 per cent oxalic acid despite the fact that, in thetest, the dilute electrolyte is favored because the water bath would effect a greater heat removal in the cell, which rose to the higher temperature.

Having thus described my invention, I claim:

1. As an electrolyte for the anodic production of oxide coatings on aluminum, an aqueous solution containing 5.0 to 9.0 per cent by weight of oxalic acid.

2. In a process of producing oxide coatings on aluminum, the step comprising making the aluminum anode in an electrolytic cell the electrolyte of which is an aqueous solution containing 5.0 to 9.0 per cent by weight of oxalic acid.

3. In a process of producing oxide coatings on aluminum, the step comprising treating the aluminum in an electrolytic cell the electrolyte of which is an aqueoussolution containing 5.0 to

9.0 per cent by weight of oxalic acid.

4. In a process of producing oxide coatings on aluminum, the steps comprising making the aluminum'an electrode in an electrolytic cell the cell, and maintaining an operating voltage below about 50 volts.

6. In a process of producing oxide coatings on aluminum, the steps comprising making the aluminum anode in an electrolytic cell the electrolyte of which is an aqueous solution containing 5.0 to 9.0 per cent by weight of oxalic acid, impressing an electric current on said electrolytic cell, and maintaining an operating voltage between about 25 to 45 volts.

- HAROLD K. WORK.' 

